Question

Because Robert was seen ____ the room, everyone thought he was the thief. 
A. to enter          B.  enter              C. Entered

Answer 1

A

试题分析：句意为：罗伯特被看见进入了房间，大家都认为他是小偷。这是一个由含有宾语+不带to的动词不定式作宾语补足语的句子变成的被动句。原句是Everyone saw Robert enter the room.
变成被动句时宾语补足语的动词不定式要加上to。故应选A。